The Mammalian sympathetic nervous system presented apparently insurmountable obstacles to both anatomists and physiologists till the publication of Gaskell’s Monograph in 1885 (1). Our knowledge of its structure and functions is now, however, placed on a much firmer basis; and it is possible to enter seriously into a consideration of its morphology and development. A. Physiology . The cells composing the ganglia in the main sympathetic chain are shown by Gaskell to be trophic only. They are neither automatic nor reflex in their action, but merely nourish the fibres which pass from them centrally or peripherally. The gray Rami communicantes spring from the ganglia and are distributed as trophic fibres to the roots and trunk of each spinal nerve and their meninges , and the bodies of the vertebræ. In some cases (e.g. fore and hind limbs) the vasomotor fibres reach their destination through these Kami. The white Rami communicantes are only found in two regions. In the Dog between the tenth and twenty-fifth spinal nerves (second dorsal—second lumbar), and in the Kami of the second and third sacral nerves. This corresponds fairly accurately with their distribution in the human subject (2). In both regions they consist of very small medullated fibres ( 1·8 µ to 2·7 µ ). In the anterior region the white Rami pass from the spinal nerves to the ganglia, and there separate into two groups; one set forms vasomotor fibres, which join the ganglia, become connected with the ganglion-cells, and are distributed peripherally as gray fibres, greatly increased in numbers. The other set does not join the ganglia, but, passing over them, forms the nerves distributed in the Splanchnics to the abdominal viscera as viscero-inhibitory fibres. In the Posterior region white Rami arise from the second and third (and in man fourth (2)) sacral nerves which pass over without joining the ganglia; becoming connected with the hypogastric plexus, they are distributed as the Nervi erigentes ( Vaso-inhibitory fibres). They are possibly also viscero-inhibitory .